(l) the Fcr-medicated endocytosis of model immune complexes has been studied using the P388D1 mouse macrophage line. Internalization increases with complex size because binding avidity increases with size and because large complexes are cleared from cell surfaces more rapidly than smaller ones. Ligand induced self aggregation greatly enhance receptor mediated uptake. About 2/3 of FcR are rapidly down modulated after binding immune complexes, the remaining 1/3 recycle between bound and unbound states. (2) The role of monomeric IgG in controlling IgG mediated effector functions has been investigated. (3) The distribution of FcR in normal human and murine cells has been examined by dual parometer flow microfluorometry. In human peripheral blood mononuclear cells, FcR distributions have been measured in cells expressing OKMI, OKT3, OKT4, and OKT8 antigens. In the mounse FcR have been measured on spleen cells which bear SigM, SigD, and IA. (4) We have adapted our techniques formeasuring FcR to quantitating Class-I histocompatibility antigens on mouse spleen cells.